Methodologies available for assessing the absorption, transport, metabolism, elimination, efficacy and toxicity of substances, such as drugs and drug candidates, span many levels of mammalian organization from in vivo studies to isolated organs or tissues, tissue slices, cultured cell types, subcellular particles, multi-enzyme complexes and molecular interactions. In practice, these complex methods result in considerable wasted time, effort and resources in many fields, particularly drug development, where drug candidates may undergo several rounds of safety and efficacy testing only to find that later testing or market experience reveals undesirable effects often with tragic consequences. For example, drugs that have been approved for human use, but later recalled due to toxicity issues include Vioxx®, Celebrex®, phexophenadine and thalidomide.
In early clinical trials, adverse benefit/risk ratios frequently cause the demise of otherwise promising pharmacologically active substances. Such events are costly and can have a profound effect on drug discovery, health care and industry stability and economics. Historically, attempts to weed-out substances having an unacceptable benefit/risk ratio have relied on in vivo non-human animal studies using several species, such as rodent species.
Limitations of toxicity studies in non-human species have long been, and still are, well recognized in the pharmaceutical industry, but short of performing toxicity studies in humans in vivo there has been no viable alternative. Attempts have been made to bridge the gap between non-human testing and humans using tissue preparations including subcellular particles, e.g., microsomes, primary cells and cells in culture, e.g., hepatocytes, and tissue slices. Although these in vitro tissue preparations generate much useful data, they have not made a significant difference in the number of drug candidates failing in clinical trials due to adverse risk issues. There is ample evidence in the literature to suggest that this is due, at least in part, to the fact that the farther the tissue preparation is from the whole organism, the greater the risk of false positives and false negatives. For example, false positives or false negatives may occur when assessing whether such test substances administered in therapeutic doses are toxic when administered alone or with other co-administered drugs. Moreover, there is no guarantee that pharmacokinetic/toxicity relationships in normal human tissues determined in vitro will be the same as in diseased human tissues in vivo.
As it is not ethical to use humans for exploratory toxicity testing, the choice has been to perform in vivo testing on a variety of non-human animal species and/or in vitro testing using human biological samples. It is recognized that the confidence in the safety and efficacy of a drug compound increases as it moves from preclinical to clinical testing. It is also recognized, however, that the dangers of unforeseen deleterious results also increase.
Thus, there is a need for improved methods for evaluating absorption, transport, metabolism, elimination, efficacy and/or toxicity of substances, such as drug candidates, that bridge the gap between in vivo non-human animal testing and human administration. In addition, there is a need, in the drug development industry, for new and improved methods of evaluating potential drug candidates early in the research and development process, and providing such evaluations to drug development companies.